This invention relates to a process for producing isophthaloyl dichloride or terephthaloyl dichloride of high purity.
Various methods have been suggested for producing the isophthaloyl dichloride or terephthaloyl dichloride, and among them, a method for producing isophthaloyl dichloride or terephthaloyl dichloride which comprises chlorinating m-xylene or p-xylene or a compound resulting from the partial chlorine-substitution of its methyl groups to form .alpha.,.alpha.,.alpha.,.alpha.',.alpha.',.alpha.'-hexachloroxylene, and reacting the resulting hexachloroxylene with isophthalic acid or terephthalic acid is especially advantageous from an economic standpoint.
The reactions in this method are schematically shown by the following reaction equations (I) and (II). ##STR1##
It is also known to produce isophthaloyl dichloride or terephthaloyl dichloride by reacting hexachloroxylene with a monocarboxylic acid, an alcohol, water, sulfur dioxide or a metal oxide such as titanium oxide. Such methods are schematically shown as follows: ##STR2##
The isophthaloyl dichloride and terephthaloyl dichloride find increasing utility as materials for the production of wholly aromatic polyamides or wholly aromatic polyesters which have recently aroused interest as excellent thermally stable polymers. Since the high purity of isophthaloyl dichloride or terephthaloyl dichloride is of paramount importance in obtaining end polymers of superior properties, it is necessary to produce isophthaloyl dichloride or terephthaloyl dichloride having the highest possible purity.
Generally, a process for preparing isophthaloyl dichloride (abbreviated IPC) or terephthaloyl dichloride (abbreviated TPC) through hexachloroxylene (abbreviated HCX) is economically advantageous. In particular, a method for producing IPC or TPC by reacting HCX with isophthalic acid or terephthalic acid is very advantageous because as shown in formulae (I) and (II), both of the starting xylene and phthalic acid can be converted to phthaloyl dichloride (PC) as an end product. However, in the step of producing HCX by chlorinating the side-chain methyl groups of xylene, by-product impurities, such as a compound resulting from the chlorination of the benzene nucleus and compounds resulting from the decomposition of the ring-chlorinated products, occur inevitably, and it is extremely difficult to remove these impurities. Phthaloyl dichlorides synthesized from HCX containing such impurities contain impurities ascribable to the aforesaid impurities present in HCX. These impurities are also difficult to remove, and at the same time, these impurities cause a marked reduction in the quality of wholly aromatic polyamides or wholly aromatic polyesters prepared from the resulting phthaloyl dichlorides. Accordingly, the removal of impurities is an important problem in the method for producing phthaloyl dichlorides through HCX.
For the production of PC of high purity, the use of HCX having least possible impurities as a starting material of the reaction of equation (II) is thought to be advantageous. This would immediately lead to an idea of purifying the HCX obtained by the reaction of equation (I) by a customary purifying method such as distillation or recrystallization. However, the HCX is extremely difficult to purify,, and neither of the distillation method nor the recrystallization method can give satisfactory purities. The fact is that since the boiling points of impurities present in HCX are quite close to that of HCX, distillation is unsatisfactory for separating these impurities from HCX. On the other hand, if HCX is purified by recrystallization, a considerable amount of the end product HCX is lost during purification because HCX has a high solubility in organic solvents and particularly, hexachloro-metaxylene has a low melting point. Thus, the recrystallization method cannot be put to practical use.
For this reason, the purification of PC synthesized through HCX generally does not rely on the purification of HCX obtained by the first step, but on the purification of PC itself from the second step. Distillation is a main method now in use for purifying PC obtained by the second step. A generally employed method for purification of PC consists of distilling the resulting PC after the second step with or without purification of HCX obtained in the first step by distillation. For example, British Pat. No. 946,491 gave consideration to the fact that the purification of HCX by distillation or recrystallization is a difficult and costly operation, and suggested a method of producing isophthaloyl dichloride which comprises distilling the crude isophthaloyl dichloride obtained by the second step, under an absolute pressure of 20 to 50 mmHg, separating a distillate forecut amounting to up to 10% of the crude isophthaloyl dichloride and collecting the balance of the distilled isophthaloyl dichloride as a product having a purity of at least 97% and a melting point of 41.degree. to 43.degree. C. Purification of impurities contained in the crude PC are difficult to remove by distillation because their boiling points are close to that of PC. PC of sufficiently high purity cannot be obtained even if the rates of cutting at the foreward distillation section and the rearward distillation section are increased at a sacrifice of the yield of PC.
Nevertheless, it has been the previous practice to purify PC by distillation, and purification by recrystallization has been considered to be disadvantageous over purification by distillation. The reason for this is that the loss of PC by a recrystallization operation is great, and a satisfactory purity cannot be obtained by only one recrystallization operation, since the amounts of impurities in PC are fairly large and the solubilities of PC and the impurities in organic solvents are similar to each other and also high. Because recrystallization can progressively increase the purity of PC by repeated operations, it is not impossible in principle to obtain PC of the desired high purity if without regard to the yield of PC, the recrystallization is repeated until the desired purity is attained. The repetition of recrystallization, however, is very disadvantageous for commercial operations because it results in a progressive decrease in yield and a markedly high cost of production. Furthermore, since an especially high purity is required of PC as a material for the production of wholly aromatic polyesters or wholly aromatic polyamides, it has previously been thought that recrystallization would be commercially unacceptable for purification of PC as such.
It is an object of this invention to provide a process by which PC of a far higher purity than PC purified by the conventional distillation method can be obtained very economically by a combination of specified manufacturing conditions for obtaining PC through HCX and the recrystallization of the resulting PC under specified conditions.
Another object of this invention is to provide a process which can remove the previous need to choose between purity and yield in the recrystallization of PC produced through HCX.
According to the process of this invention, PC of very high purity can be obtained by only one recrystallizing operation.